Cryogenic vacuum pumps, or cryopumps, currently available generally follow a common design concept. A low temperature array, usually operating in the range of 4 to 25K, is the primary pumping surface. This surface is surrounded by a higher temperature radiation shield, usually operated in the temperature range of 60 to 130K, which provides radiation shielding to the lower temperature array. The radiation shield generally comprises a housing which is closed except at a frontal array positioned between the primary pumping surface and a work chamber to be evacuated.
In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through that array and into the volume within the radiation shield and condense on the lower temperature array. A surface coated with an adsorbent such as charcoal or a molecular sieve operating at or below the temperature of the colder array may also be provided in this volume to remove the very low boiling point gases such as hydrogen. With the gases thus condensed and/or adsorbed onto the pumping surfaces, only a vacuum remains in the work chamber.
In systems cooled by closed cycle coolers, the cooler is typically a two-stage refrigerator having a cold finger which extends through the rear or side of the radiation shield. High pressure helium refrigerant is generally delivered to the cryocooler through high pressure lines from a compressor assembly. Electrical power to a displacer drive motor in the cooler is usually also delivered through the compressor.
The cold end of the second, coldest stage of the cryocooler is at the tip of the cold finger. The primary pumping surface, or cryopanel, is connected to a heat sink at the coldest end of the second stage of the cold finger. This cryopanel may be a simple metal plate or cup or an array of metal baffles arranged around and connected to the second-stage heat sink. This second-stage cryopanel also supports the low temperature adsorbent.
The radiation shield is connected to a heat sink, or heat station, at the coldest end of the first stage of the refrigerator. The shield surrounds the second-stage cryopanel in such a way as to protect it from radiant heat. The frontal array is cooled by the first-stage heat sink through the side shield or, as disclosed in U.S. Pat. No. 4,356,701, through thermal struts.
After several days or weeks of use, the gases which have condensed onto the cryopanels, and in particular the gases which are adsorbed, begin to saturate the cryopump. A regeneration procedure must then be followed to warm the cryopump and thus release the gases and remove the gases from the system. As the gases evaporate, the pressure in the cryopump increases, and the gases are exhausted through a relief valve. During regeneration, the cryopump is often purged with warm nitrogen gas. The nitrogen gas hastens warming of the cryopanels and also serves to flush water and other vapors from the cryopump. By directing the nitrogen into the system close to the second-stage array, the nitrogen gas which flows outward to the exhaust port minimizes the movement of water vapor from the first array back to the second-stage array. Nitrogen is the usual purge gas because it is inert and is available free of water vapor. It is usually delivered from a nitrogen storage bottle through a fluid line and a purge valve coupled to the cryopump.
After the cryopump is purged, it must be rough pumped to produce a vacuum about the cryopumping surfaces and cold finger to reduce heat transfer by gas conduction and thus enable the cryocooler to cool to normal operating temperatures. The rough pump is generally a mechanical pump coupled through a fluid line to a roughing valve mounted to the cryopump.
Control of the regeneration process is facilitated by temperature gauges coupled to the cold finger heat stations. Thermocouple pressure gauges have also been used with cryopumps but have generally not been recommended because of a potential of igniting gases released in the cryopump by a spark from the current-carrying thermocouple. The temperature and/or pressure sensors mounted to the pump are coupled through electrical leads to temperature and/or pressure indicators.
Although regeneration may be controlled by manually turning the cryocooler off and on and manually controlling the purge and roughing valves, a separate regeneration controller is used in more sophisticated systems. Leads from the controller are coupled to each of the sensors, the cryocooler motor and the valves to be actuated.
A cryopump comprises a cryogenic refrigerator, a gas condensing cryopanel cooled by the refrigerator, at least one temperature sensor coupled to the cryopanel and an electrically actuated valve, such as roughing valve, adapted to remove gases from the cryopump.
In accordance with an embodiment of the present invention, after determining that a power fail recovery mode is on, and that the temperature is below a setpoint, a cryopump may be automatically cooled. If the power fail recovery mode is on, and the temperature is above the setpoint, a regeneration cycle may be started automatically.
If, on the other hand, the power fail recovery mode is off, the cryopump may be maintained in an off state.
According to another embodiment of the present invention, when a command to turn on a thermocouple (TC) gauge is received, where the TC gauge senses pressure within the cryopump, it is first determined whether there is a sufficient amount of gas to ignite. For example, either a temperature being below some setpoint, or the cryopump having already been purged may indicate that there is an insufficient amount of gas to ignite. When the command is received, the TC gauge is turned on only if there is not a sufficient amount of gas to ignite.
According to yet another embodiment of the present invention, when a command to turn on a roughing valve is received, a warning may be displayed if the cryopump is on. The roughing valve may be turned on only upon receiving an additional command to turn on the roughing valve.
According to still another embodiment of the present invention, whether a temperature-sensing diode is faulty, for example if it reads out of its normal range, is detected. If the diode is faulty, temperature control is automatically turned off.
According to yet another embodiment of the present invention, during a regeneration cycle, a purge test is performed. If the purge test passes, heaters are turned on. If the purge test fails, the heaters are turned on only if the temperature reaches a threshold within a predetermined period. Otherwise, the regeneration is aborted.